This invention relates to an air-fuel mixture ratio control device for controlling the air-fuel ratio of a fuel mixture to be supplied to an engine with the aid of an exhaust gas concentration signal.
Generally, a method for reducing the noxious components in the exhaust gas from an automobile comprises feeding back information from an exhaust gas sensor to control the air-fuel ratio of the mixture drawn into the engine.
In this method, the components (e.g., CO, O.sub.2, CO.sub.2, etc). in the exhaust gas are detected by an exhaust gas sensor 1 provided in the exhaust pipe 2, as shown in FIG. 1. A control circuit 3 produces a control signal, containing a proportional element and an integrated element, from a deviation signal which depends upon the deviation between a predetermined air-fuel ratio and the actual air-fuel ratio detected by the sensor. An actuator 4 is actuated by the control signal from the control circuit 3 to control the quantity of fuel and air in a fuel supplying device 5 and thereby maintain the air-fuel ratio at a desired value, e.g., a value at which an exhaust gas purifier 7 functions most effectively to reduce noxious components in the exhaust gas.
In the air-fuel ratio control device of this type, a change in the amount of air intake is accompanied by a change in the degree of influence upon the base air-fuel ratio (the air-fuel ratio when no feedback control is applied) of the mixture supplied from the air-fuel ratio control device (e.g., the carburetor), and the control air-fuel ratio for feedback control of the air and fuel. Hence, the control lever of the feedback control varies as the operating condition of the engine changes. Since the control signal generally contains an integrated element, it takes time for the control lever to shift from the level of idling to that of acceleration such as when the engine is accelerated from the idling condition. During this transition period, accurate control of the fuel ratio cannot be achieved, allowing a large amount of noxious gases to be exhausted.
It has therefore been considered to increase the control speed to reduce the time lag as much as possible. However, since there is a time lag after the air-fuel ratio has been changed in the fuel supplying device until the output of the exhaust gas sensor changes, an increase in the control speed will likely result in hunting under such operating conditions as idling when the quantity of intake air is small.